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Aircraft Parts, FAA, FAA Design Approval, Manufacturing, PMA, Regulatory

New FAA Standards: Fatigue Tolerance Evaluation of Metallic Structure NPRM

The FAA published a Notice of Proposed Rule Making (NPRM) on March 12 that would implement a new fatigue tolerance standard for transport category rotorcraft (Part 29 rotorcraft).

The FAA’s NPRM regarding fatigue tolerance evaluations of metallic structures states that it seeks to de-emphasize the use of specific methodologies in the fatigue tolerance evaluations of rotorcraft.  But in an interesting twist, the proposed rule would specifically require applicants to submit their fatigue tolerance evaluation methodologies to the Administrator for approval.

The current version of § 29.571 requires that the applicant use one of three evaluation methods in order to demonstrate fatigue tolerance:

  • Flaw tolerant safe-life evaluation;
  • Fail-safe (residual strength after flaw growth) evaluation;
  • Safe-life evaluation.

The positive spin on the new version of the rule is that it permits applicants to use other evaluation methods to show fatigue tolerance.  The negative is that it provides no standards whatsoever that the FAA will use for its approval of the applicant’s proposed methods – it offers some elements that need to be included but it does not state (nor suggest) that the FAA will be wiling to approve a methodology that merely addresses those elements that are required to be included (at those included elements, alone, appear to be inadequate to meet current FAA desires for fatigue analysis).

Because compliance testing methods are usually described as proprietary, it is possible for the FAA to approve one applicant’s proposal to use a method that has already been rejected from another applicant (or vice versa), providing a real opportunity for the FAA to impose disparate standards on different applicants with almost no opportunity for the applicants to realize that they have been treated differently.  In fact, it is possible, under the new rule, for an applicant to use one of the three methods found under the “old rule” and for that method to be rejected by the FAA, because the FAA has no objective standards for what it must approve under the new rule.

The proposed § 29.571 requires an identification of all threats to all principal structural elements (PSE).  A PSE is defined as any structural element that contributes significantly to the carriage of flight or ground loads. The threat assessment has to include all probable locations, types, and sizes of damage.  It also has to take into account fatigue, environmental effects, intrinsic and discrete flaws, or accidental damage that may occur during manufacture or operation.  Taking into account all locations, types, and sizes of accidental damage seems like a significantly increased burden.

The fatigue analysis must support the inspection and retirement times established in the airworthiness limitations section, and the inspections and retirement times must be established (based on the analysis of the new section) to avoid catastrophic failure.  The language of the rule, as drafted, appears to require that inspection intervals and life limits be established to prevent catastrophic failure in the event of accidental damage.

Current Regulation

Proposed Regulation

§ 29.571 Fatigue evaluation of structure.

(a) General. An evaluation of the strength of principal elements, detail design points, and fabrication techniques must show that catastrophic failure due to fatigue, considering the effects of environment, intrinsic/discrete flaws, or accidental damage will be avoided. Parts to be evaluated include, but are not limited to, rotors, rotor drive systems between the engines and rotor hubs, controls, fuselage, fixed and movable control surfaces, engine and transmission mountings, landing gear, and their related primary attachments. In addition, the following apply:

(1) Each evaluation required by this section must include—

(i) The identification of principal structural elements, the failure of which could result in catastrophic failure of the rotorcraft;

(ii) In-flight measurement in determining the loads or stresses for items in paragraph (a)(1)(i) of this section in all critical conditions throughout the range of limitations in §29.309 (including altitude effects), except that maneuvering load factors need not exceed the maximum values expected in operations; and

(iii) Loading spectra as severe as those expected in operation based on loads or stresses determined under paragraph (a)(1)(ii) of this section, including external load operations, if applicable, and other high frequency power cycle operations.

(2) Based on the evaluations required by this section, inspections, replacement times, combinations thereof, or other procedures must be established as necessary to avoid catastrophic failure. These inspections, replacement times, combinations thereof, or other procedures must be included in the airworthiness limitations section of the Instructions for Continued Airworthiness required by §29.1529 and section A29.4 of appendix A of this part.

(b) Fatigue tolerance evaluation (including tolerance to flaws). The structure must be shown by analysis supported by test evidence and, if available, service experience to be of fatigue tolerant design. The fatigue tolerance evaluation must include the requirements of either paragraph (b)(1), (2), or (3) of this section, or a combination thereof, and also must include a determination of the probable locations and modes of damage caused by fatigue, considering environmental effects, intrinsic/discrete flaws, or accidental damage. Compliance with the flaw tolerance requirements of paragraph (b)(1) or (2) of this section is required unless the applicant establishes that these fatigue flaw tolerant methods for a particular structure cannot be achieved within the limitations of geometry, inspectability, or good design practice. Under these circumstances, the safe-life evaluation of paragraph (b)(3) of this section is required.

(1) Flaw tolerant safe-life evaluation. It must be shown that the structure, with flaws present, is able to withstand repeated loads of variable magnitude without detectable flaw growth for the following time intervals—

(i) Life of the rotorcraft; or

(ii) Within a replacement time furnished under section A29.4 of appendix A to this part.

(2) Fail-safe (residual strength after flaw growth) evaluation. It must be shown that the structure remaining after a partial failure is able to withstand design limit loads without failure within an inspection period furnished under section A29.4 of appendix A to this part. Limit loads are defined in §29.301(a).

(i) The residual strength evaluation must show that the remaining structure after flaw growth is able to withstand design limit loads without failure within its operational life.

(ii) Inspection intervals and methods must be established as necessary to ensure that failures are detected prior to residual strength conditions being reached.

(iii) If significant changes in structural stiffness or geometry, or both, follow from a structural failure or partial failure, the effect on flaw tolerance must be further investigated.

(3) Safe-life evaluation. It must be shown that the structure is able to withstand repeated loads of variable magnitude without detectable cracks for the following time intervals—

(i) Life of the rotorcraft; or

(ii) Within a replacement time furnished under section A29.4 of appendix A to this part.

§ 29.571 Fatigue Tolerance Evaluation of Metallic Structure.

(a) A fatigue tolerance evaluation of each principal structural element (PSE) must be performed, and appropriate inspections and retirement time or approved equivalent means must be established to avoid catastrophic failure during the operational life of the rotorcraft. The fatigue tolerance evaluation must consider the effects of both fatigue and the damage determined in paragraph (e)(4) of this section. Parts to be evaluated include PSEs of the rotors, rotor drive systems between the engines and rotor hubs, controls, fuselage, fixed and movable control surfaces, engine and transmission mountings, landing gear, and their related primary attachments.

(b) For the purposes of this section, the term—

Catastrophic failure means an event that could prevent continued safe flight and landing.

Principal Structural Element (PSE) means a structural element that contributes significantly to the carriage of flight or ground loads, and the fatigue failure of that structural element could result in catastrophic failure of the aircraft.

(c) The methodology used to establish compliance with this section must be submitted and approved by the Administrator.

(d) Considering all rotorcraft structure, structural elements, and assemblies, each PSE must be identified.

(e) Each fatigue tolerance evaluation required by this section must include:

(1) In-flight measurements to determine the fatigue loads or stresses for the PSEs identified in paragraph (d) of this section in all critical conditions throughout the range of design limitations required in § 29.309 (including altitude effects), except that maneuvering load factors need not exceed the maximum values expected in operations.

(2) The loading spectra as severe as those expected in operations based on loads or stresses determined under paragraph (e)(1) of this section, including external load operations, if applicable, and other high frequency power-cycle operations.

(3) Takeoff, landing, and taxi loads when evaluating the landing gear and other affected PSEs.

(4) For each PSE identified in paragraph (d) of this section, a threat assessment which includes a determination of the probable locations, types, and sizes of damage, taking into account fatigue, environmental effects, intrinsic and discrete flaws, or accidental damage that may occur during manufacture or operation.

(5) A determination of the fatigue tolerance characteristics for the PSE with the damage identified in paragraph (e)(4) of this section that supports the inspection and retirement times, or other approved equivalent means.

(6) Analyses supported by test evidence and, if available, service experience.

(f) A residual strength determination is required to establish the allowable damage size. In determining inspection intervals based on damage growth, the residual strength evaluation must show that the remaining structure, after damage growth, is able to withstand design limit loads without failure within its operational life.

(g) The effect of damage on stiffness, dynamic behavior, loads, and functional performance must be considered.

(h) Based on the requirements of this section, inspections and retirement times or approved equivalent means must be established to avoid catastrophic failure. The inspections and retirement times or approved equivalent means must be included in the Airworthiness Limitations Section of the Instructions for Continued Airworthiness required by Section 29.1529 and Section A29.4 of Appendix A of this part.

(i) If inspections for any of the damage types identified in paragraph (e)(4) of this section cannot be established within the limitations of geometry, inspectability, or good design practice, then supplemental procedures, in conjunction with the PSE retirement time, must be established to minimize the risk of occurrence of these types of damage that could result in a catastrophic failure during the operational life of the rotorcraft.

Overall, the NPRM proposes to change the landscape for parties performing fatigue tolerance evaluation.

Comments are due to the FAA on or before June 10, 2010.  MARPA would like to hear your comments on this proposal, in order to make sure that the Association’s comments to the docket accurately reflect your concerns.

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About Jason Dickstein

Mr. Dickstein is the President of the Washington Aviation Group, a Washington, DC-based aviation law firm. He represents several aviation trade associations, including the Aviation Suppliers Association, the Aircraft Electronics Association, the Aircraft Fleet Recycling Association and the Modification and Replacement Parts Association.

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